When using Mockito.spy() to mock a method being called by another method, why do I get a UnnecessaryStubbingException?

Question

I have the following class-

public class A {
    int a;

    public A() {
        this.a = 67;
    }
    
    public int get_a() {
        return this.a;
    }
    
    public boolean is_a_even() {
        if(this.get_a() % 2 == 0) {
            return true;

        } else{
            return false;
        }
    }
}

I want to mock the get_a() function, so that it returns a value that I want it to, so I wrote the following test-

@ExtendWith(MockitoExtension.class)
class ATest {

    @Test
    public void testing_if_call_from_another_function_is_mocked() {
        A obj = new A();

        A mock = Mockito.spy(obj);

        Mockito.doReturn(2).when(mock).get_a();

        assertTrue(obj.is_a_even());

    }
}

But when I run this, I get the following exception-

org.mockito.exceptions.misusing.UnnecessaryStubbingException: 
Unnecessary stubbings detected.
Clean & maintainable test code requires zero unnecessary code.
Following stubbings are unnecessary (click to navigate to relevant line of code):
  1. -> at com.example.demo.ATest.testing_if_call_from_another_function_is_mocked(ATest.java:19)
Please remove unnecessary stubbings or use 'lenient' strictness. More info: javadoc for UnnecessaryStubbingException class.

    at org.mockito.junit.jupiter.MockitoExtension.afterEach(MockitoExtension.java:186)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeAfterEachCallbacks$11(TestMethodTestDescriptor.java:253)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeAllAfterMethodsOrCallbacks$12(TestMethodTestDescriptor.java:269)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeAllAfterMethodsOrCallbacks$13(TestMethodTestDescriptor.java:269)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeAllAfterMethodsOrCallbacks(TestMethodTestDescriptor.java:268)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeAfterEachCallbacks(TestMethodTestDescriptor.java:252)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:137)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:65)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:108)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:88)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:54)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:67)
    at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:52)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:96)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:75)
    at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:71)
    at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
    at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:221)
    at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)

Does this mean that the method call to get_a() is not getting mocked, and if so, how do I mock a method which is going to be called from another method?

Answer 1

You get UnnecessaryStubbingException as mock.get_a() is never called.

This is because you call

assertTrue(obj.is_a_even());

Instead of:

assertTrue(mock.is_a_even());

Thus in is_a_even method this points to obj , not to mock

Other remarks:

• stick to JavaBean naming conventions - getA() instead of get_a() . This is widespread and a lot of tools rely on it.
• substituting a part of system under test is an advanced tool - use it sparingly. If possible, mock collaborators of SUT, not SUT itself.

Answer 2

Let us dig a little bit deeper to understand what is happening and why.

When we call

A mock = Mockito.spy(obj);

mockito creates a wrapper-object, that we can use to spy on an object. We use this spy to then mock the call we want to mock:

Mockito.doReturn(2).when(mock).get_a();

Notice that the original obj stays unchanged. In particular, its behaviour stays unchanged. If we use obj , it will behave as if we had never created a spy.

Hence, if we call

obj.is_a_even()

Our mock will not be used. To get the desired behaviour, we need to call

mock.is_a_even()

---

The proper solution, however, would be to make the class testable, eg by providing a constructor that allows to set the value of a , eg:

public class A {
    private final int a;

    public A(int a) {
        this.a = a;
    }

    public A() {
        this(67);
    }

    ...
}

With this setup, we can write the same test without the need of any mock.

---

Three remarks:

• We should respect encapsulation, ie set properties private and make them accessible only through accessor methods.

• We should favor

  Mockito.when(mock.get_a()).thenReturn(2);

  over

  Mockto.doReturn(2).when(mock).get_a();

  since the former guarantees type-safety at compile-time, the later does not.

• In Java, method names should be written in lowerCamelCase instead of snake_case ( get_a() -> getA() , is_a_even() -> isAEven() ). I would also recommend to find better names, for the field a as well as for the two methods.